This invention relates, in general, to amplifiers and more particularly to a temperature stabilized common emitter amplifier.
Typically a common emitter amplifier is temperature stabilized by adding a resistor in series with the emitter of the transistor. The gain of the amplifier can then be calculated by dividing the resistance of this emitter resistor into the resistance of the collector load resistor of the amplifier, provided that the emitter resistor is much greater than the internal emitter resistance of the transistor and the collector load resistor is much smaller than the internal collector resistance of the transistor. Of course the smaller the resistance of the resistor in series with the emitter, the larger the gain. If the resistor is made too small though then an undesired effect occurs. The internal emitter resistance of the transistor becomes a dominant factor in calculating gain, and as is well known, the internal emitter resistance is directly proportional to temperature. Therefore, it is desirable to keep the external resistor in series with the emitter large enough so that the internal emitter resistance does not assume an overbearing factor in controlling gain. The collector load resistor cannot arbitrarily be increased to increase the gain due to the low power supply voltage normally associated with integrated circuits. In addition, increasing the collector load resistance would change the quiescent operating current of the amplifier if voltage feedback is provided because the voltage feedback requires the quiescent output voltage to be the same for different values of collector load resistance. If the collector load resistance is increased for a given value of supply voltage, V.sub.CC, and of feedback voltage, then of course, the emitter current will be lower and, as a result, the internal emitter resistance will increase. One scheme widely used in integrated circuits to increase the gain of a common emitter amplifier is to replace the collector load resistor with a current source. This works well for many applications, however, such a scheme is not satisfactory for applications requiring closely controlled gain of the common emitter amplifier since gain is not too closely controlled when the collector load resistor is replaced by a current source.
Accordingly, it is an object of the present invention to provide an improved common emitter amplifier.
Another object of the present invention is to provide a common emitter amplifier that has temperature stabilization and closely controlled gain.
Yet another object of the present invention is to provide a common emitter amplifier having a current source in parallel with the collector load resistor.
A further object is to provide a common emitter amplifier, having a collector load resistor, with additional emitter current to decrease the effects, of variations of the internal emitter resistance of the transistor with temperature, on gain of the amplifier.